[scottw]

"Population declines following economic collapse" should be "Population declines following sustained drop in birthrate." The population is going to decline regardless of what the future economy does.

[kragen]

I'm not very familiar with their model, but if the birthrate asymptotes to above the replacement rate, or goes back up, the population won't decline. So the economy might be a factor.

[schiffern]

scottw is referring to the well-studied demographic transition.

http://en.wikipedia.org/wiki/Demographic_transition

[kragen]

I knew that already.

[toomuchtodo]

TL;DR The world is getting older, hence less babies born.

[toomuchtodo]

Downvote? For quickly summarizing a wikipedia link? I guess we all have time to waste skimming wikipedia articles, don't we?

[schiffern]

The problem is that it's wrong.

A better summary would be, "The world is getting richer, hence lower infant mortality, hence birth rates drop."

[toomuchtodo]

The world is graying extremely quickly (Japan, Western Europe, the US, and soon China).